Heterojunction bipolar transistors (HBTs) are attractive devices for such applications as amplifiers for personal communications and radar systems, and as switching devices in products such as analog-to-digital converters. The maximum frequency of oscillation of GaAs-based HBTs can be five times higher than that of silicon bipolar transistors. In addition, since HBTs are vertical devices, they possess a superior packing density as compared to GaAs MESFETs. Despite these advantages over silicon bipolar and even GaAs FET technologies, GaAs-based HBTs have been shown to suffer from a lower current gain than should be expected from this type of device.
A typical prior-art mesa HBT is shown in FIG. 1a. In mesa HBTs the base metal contact 10 is placed as close to the emitter contact 12 as possible in order to reduce the extrinsic base resistance. The extrinsic base is the portion of the base layer 14 that lies outside the bounds of the emitter layer 16. In general, the closer the base contacts 10 are to the emitter 16, the lower the base resistance. While this approach can lower the base resistance of a transistor, it can also result in reduced HBT current gains. The base ohmic contact 10 represents an infinite surface recombination velocity for carriers injected from the emitter 16, resulting in an increase in the base current and a reduction in current gain. It has been shown that the HBT current gain decreases as the lateral spacing between the base contact 10 and the emitter contact 12 is decreased below approximately 1 .mu.m. See Won-Seong Lee, et al., Effect of Emitter-Base Spacing on the Current Gain of AlGaAs/GaAs Heterojunction Bipolar Transistors, IEEE Electron Device Letters, Vol. 10, No. 5, May 1989, and Yoshiko Someya, et al., Two-Dimensional Analysis of the Surface Recombination Effect on Current Gain for GaAlAs/GaAs HBTs, IEEE Transactions on Electron Devices, Vol. 35, No. 7, July 1988.
The transistor current gain is also dependent upon base layer doping and thickness. For high current gains, the base layer is generally lightly doped and very thin. This configuration contrasts with the need to have low base resistance, which generally requires high base layer doping and a thicker base layer. The practice of moving the base contact metallization further from the emitter contact for improved current gains is not suitable in transistors having lightly doped base layers because of the unacceptable increase in extrinsic base resistance. A compromise is usually made between these conflicting requirements.
In the past, efforts have been made to increase transistor current gain by minimizing the recombination at the extrinsic base surface 18. In these approaches a physical or chemical passivation treatment is applied to the surface of the base layer of the HBT. See O. Nakajima, et al., "Emitter-Base Junction Size Effect on Current Gain H.sub.fe of AlGaAs/GaAs Heterojunction Bipolar Transistors", Japanese Journal of Applied Physics, Vol. 24, No. 8, pp. L596-L598, Aug. 1985; R. J. Malik, et al., "Submicron Scaling of AlGaAs/GaAs Self-aligned Thin Emitter Heterojunction Bipolar Transistors with Current Gain Independent of Emitter Area", Electronics Letters, Vol. 25, No. 17, pp. 1175-1177, Aug. 17, 1989; S. Tiwari, et al., "Surface Recombination in GaAlAs/GaAs Heterostructure Bipolar Transistors", Journal of Applied Physics, Vol. 64, No. 10, pp. 5009-5012, Nov. 15, 1988. In Lee, supra, a ledge of AlGaAs 30, part of the emitter 26 of the HBT, is left between the emitter 26 and base contact 20 to passivate the surface of the base layer 24 as shown in FIG. 1b. The wider this AlGaAs passivating ring (up to about 1 .mu.m in width), the higher the HBT current gain. However, this passivating AlGaAs ring increases the space between the base 20 and emitter 22 contacts with a corresponding increase in the base extrinsic resistance. Consequently, there is a need in the industry for transistors, and processes for making such transistors, that address these shortcomings of the prior art